Over one million Americans suffer from myocardial infarction each year. Although most patients survive the initial event, the physiologic regeneration in the adult heart is grossly inadequate to compensate for the severe loss of myocardium. Some higher organisms, such as zebrafish, are capable of complete and sufficient regeneration of the myocardium following injury. Furthermore, mice are capable of regenerating their hearts early after birth. Although adult humans and adult mice lack this cardiac regeneration potential, there is great interest in understanding how regeneration can occur in the heart so that we can activate this process in humans to better treat patients following myocardial infarction. The newt limb can completely regenerate after amputation, and this regeneration depends critically upon nerve activity. Both mechanical ablations as well as the classic non-selective muscarinic antagonist, atropine, abolish the dramatic regenerative capacity of severed limbs in newts, demonstrating that nerve activity is crucial for at least some types of organ regeneration. [The proposed research tests the hypothesis that cardiac regeneration in the neonatal mouse is also dependent upon nerve signaling.] Our preliminary data demonstrate that following exposure to atropine, zebrafish that had undergone apical resection were unable to regenerate their hearts below the resected plane. Essential proteins required for cell cycle re-entry and proliferation in cardiomyocyte regeneration were significantly downregulated in the ventricular cardiomyocytes of atropine treated hearts compared to control. Taken together, these data strongly implicate a role for nerves in cardiac regeneration following injury. Recent experiments have shown that shortly after birth, neonatal mice can also regenerate their hearts following injury. This striking phenomenon indicates that the mammalian system retains the ability to repair itself at least during this critical period. In order to invesigate the role that cholinergic nerves play in mammalian cardiac regeneration we plan to study the effects of atropine and gene knockout of the cholinergic specific neurotrophic factor, neurturin, on cardiac regeneration in the neonatal mouse. [Futhermore, nerve-dependent limb regeneration in the newt is partially mediated by the secreted protein Agr2. As in the newt limb, our preliminary data demonstrates that Agr2 can partially rescue cardiac regeneration in denervated zebrafish. Here, we test the hypothesis that Agr2 administration rescues cardiac regeneration in the neonatal mouse.] Utilizing the neonatal mouse model of cardiac regeneration will be a powerful supplement to our preliminary zebrafish experiments. The mouse heart more closely resembles the human heart and therefore can provide important insight into the cardiac regenerative potential in humans. The insights from the proposed research are critically important for reaching the goal of understanding how the heart can be regenerated following injury.